An increasing number of light fixtures utilize LEDs as light sources due to their lower energy consumption, smaller size, improved robustness, and longer operational lifetime relative to conventional filament-based light sources. Conventional LEDs emit quasi-monochromatic radiation ranging from infrared through the visible spectrum to ultraviolet. Conventional LEDs emit light at a particular wavelength, ranging from, for example, red to blue or ultraviolet (UV) light. However, for purposes of general illumination, the relatively narrow spectral width of light emitted light by LEDs is generally converted to broad-spectrum white light.
Traditionally, there are two ways of obtaining white light from LEDs. One approach utilizes two or more LEDs operating at different wavelengths, where the different wavelengths are chosen such that their combination appears white to the human eye. For example, one may use LEDs emitting in the red, green, and blue wavelength ranges. Such an arrangement typically requires careful control of the operating currents of each LED, such that the resulting combination of wavelengths is stable over time and different operating conditions, for example temperature. Because the different LEDs may be formed from different materials, different operating parameters may be required for the different LEDs; this complicates the LED circuit design. Furthermore, such systems typically require some form of light combiner, diffuser or mixing chamber, so that the eye sees white light rather than the distinct colors of each of the different LEDs. Such light-mixing systems typically add cost and bulk to lighting systems and may reduce their efficiency.
White light may also be produced in LED-based systems for general illumination via the utilization of wavelength-conversion materials (also called light-conversion materials) such as phosphors, sometimes called phosphor-converted LEDs. For example, an LED combined with a wavelength-conversion material generates white light by combining the short-wavelength radiant flux (e.g., blue light) emitted by the semiconductor LED with long-wavelength radiant flux (e.g., yellow light) emitted by the wavelength conversion material. The chromaticity (or color), color temperature, and color-rendering index are determined by the relative intensities of the component colors. For example, the light color may be adjusted from “warm white” with a correlated color temperature (CCT) of 2700 Kelvin or lower to “cool white” with a CCT of 6500 Kelvin or greater by varying the type or amount of phosphor material. White light may also be generated solely or substantially only by the light emitted by the one or more wavelength conversion materials.
In isolation, bare LED dies generally exhibit a Lambertian luminous intensity distribution pattern, as shown in FIG. 1A, that is a consequence of the light being uniformly emitted from a planar surface. (That is, the projected area of its light-emitting region decreases according to the cosine of the viewing angle with respect to the surface normal.)
The wavelength-conversion material is generally one or more phosphor particles. Such particles emit with a substantially isotropic distribution. In a phosphor-converted LED, the phosphor particles are generally embedded into a transparent matrix, for example a silicone, and typically have a substantially hemispherical shape surrounding the die with the die positioned at the equator and in the center of the hemisphere. The hemispherical shape is used because it generally results in relatively high light extraction efficiency because of reduced total internal reflection (TIR) at the phosphor/air interface. The intensity distribution of isotropic emitting phosphor particles in a hemispherical transparent matrix is shown in FIG. 1B.
As may be seen by comparing FIGS. 1A and 1B, the intensity distributions of a bare-die LED and embedded phosphor particles are different. This difference results in the chromaticity of the combined light varying with viewing angle, resulting in a non-uniform color distribution as seen by the human eye. For example, a phosphor-coated blue LED may be typically perceived as being cool white when viewed head-on, but warm white when viewed obliquely. Thus, while the hemispherical shape provides relatively high efficiency, it suffers from relatively poor color temperature uniformity with angle.
In order to mitigate the relatively poor angular color uniformity of conventional phosphor-converted LEDs, illumination systems incorporating such phosphor-converted LEDs often require additional elements, such as diffusers, mixing chambers, or the like, to homogenize the color characteristics. Such homogenization often degrades the light-intensity distribution pattern, however, resulting in the need for secondary optics to attempt to re-establish the desired light-intensity distribution pattern. The addition of these elements typically requires undesirable additional space or volume, adds cost and expense, and reduces output efficiency.
Accordingly, there is a need for structures, systems and procedures enabling LED-based illumination systems to generate uniform color distribution of emitted light and operate with high extraction efficiency while utilizing low-cost integration of phosphors with the LEDs.